The present invention relates to a hold-down anchor device for securing the reinforcing strands of a prestressed concrete building element in a mold while the strands are tensioned, the mold filled with concrete, and the concrete allowed to set. It is known that the strand characteristics of a concrete building element, such as an I-beam girder, may be improved by embedding strands or cables within the concrete. Also, it is known to tension the strands prior to pouring the concrete, thereby increasing the strength characteristics of the element.
A typical strand configuration in an I-beam girder is one in which the strands extend inwardly and downwardly from the upper portion of one end of the girder and pass on an incline through the web portion and into the base portion of the girder where the strands are engaged by a hold-down device. The strands then extend horizontally along the base portion to a second hold-down device from which they extend upwardly and outwardly into the head portion of the opposite end of the girder.
The reinforcing strands of such an element are placed under a great deal of tension, and a hold-down device used with a number of such strands, therefore, is required to handle large static forces. Typically hold-down devices are designed to handle large lift forces, e.g. 5000 pounds per strand, which tend to pull the strands from the girder base portion of an element mold. Lateral forces creating a bending moment about an anchor bolt securing the hold-down device to the wall of the mold will quite often damage the bolt or the hold-down device, however. In order to overcome this problem, a swiveling hold-down, such as shown in U.S. Pat. No. 3,854,258, issued Dec. 17, 1974 to Colado et al, has been used. With such a design, the hold-down may swivel at its base to assume an inclined position in which the upward thrust on the cables is along the length of the hold-down. That is, the angle of incidence of each cable with respect to the hold-down equals the angle of emergence so that the force moment which is created about the anchor bolt of the hold-down device is minimized.
A variation of this design is shown in U.S. Pat. No. 3,831,331, issued Aug. 27, 1974, to Colado. In that device, a metal cylinder has a threaded opening for engaging an anchor bolt and also provides a pivot for the hold-down. A central opening in the cylinder beneath the pivot allows an additional cable to be secured.
One disadvantage of previously known pivoting hold-down devices has been that the reinforcing strands could not be positioned as close to the mold wall as might be desired. Specifically, it is desired to maintain a predetermined minimum distance in the finished building element between the surface of the element and any metallic member, either a strand or a part of the hold-down unit. This results in an element having superior fire resistant characteristics, as well as substantial immunity from rusting. With previously known hold-down devices, however, maintaining this minimum distance has resulted in the cables being positioned a substantially greater distance from the surface of the building element.